The present invention relates to a circuit for producing analog signals of primary colors RGB (red, green, blue) of a television signal from its digital luminance Y and chrominance CR and CB components.
The invention particularly applies to the reception of television signals transmitted by satellite and to the distribution of television pictures on a local network. These fields require means for the flexible filtering of the components received to take account of variations in time and the location of the reception conditions, particularly in the case of satellite transmission, and the different possible picture sizes and consequently the different possible sampling frequencies of the digital components.
In the field of color television, the acquisition or scanning of a scene is carried out by means of a camera equipped with three analyzer tubes, each corresponding to one of the three primary colors, namely red, green and blue. Conventionally the electric signals produced by these tubes are combined in such a way as to reduce the total band width required for the transmission of said television signal. For this purpose, a luminance signal Y, proportional to the monochrome brightness, and two chrominance signals, signals CR and CB are produced from the red, green and blue signals CR and CB respectively equal R-Y and B-Y normalized to have a range from 0 to 1.
For some years now developments have taken place in the sense of digitizing the luminance and chrominance components produced by the camera, with a view to the digital transmission of the television signal to the receiver and optionally a digital processing of said signal in the receiver. This development has been linked with the appearance of fast VLSI circuits making it possible to replace, at reduced cost and for greater reliability, an important part of the analog functions by their digital equivalent.
Transmission in the form of digital components of the television signal has the advantage of improving the reception quality, accompanied by a greater noise immunity of the transmitted signal. This digitization also permits a greater flexibility of use, particularly for the storage on tape of these signals in broadcasting studios or for the manipulation of said signals ("incrustations", special effects and the like). Finally, said digitization makes it possible to code a scene independently of the standard television system used, such as the NTSC, PAL and SECAM systems.
A circuit for producing primary color signals from digital components of a television signal is known. This circuit comprises a digital-analog converter for carrying out an analog conversion of the digital components, a low-pass filter for attenuating the duplications of the spectra of the components introduced by digitizing said components and a dematrixing member for converting the analog luminance and chrominance signals into primary color signals.
In this circuit, the digital components are firstly converted into analog signals and are then combined to produce the primary color signals. This method suffers from disadvantages. Thus, it is difficult to obtain in a simple manner a linear phase analog low-pass filter, although this feature is essential in video. It is also impossible to modify the template of an analog filter, which limits its flexibility in an environment where there can be variations to the coding parameters (picture size, sampling frequency) and the signal reception conditions. However, with a digital filter, the template of the filter undergoes a simple homothetic transformation as a function of the sampling frequency value, making it possible for the same circuit to be able to process, without any modification, television signals present in the form of digital components, no matter what the values of the sampling frequencies of the chrominance and luminance signals, provided that they remain in the same ratio.
Moreover, the coefficients of the dematrixing member are realized with the aid of resistors. The regulation of these coefficients requires a very careful sorting of the resistors, if it is wished to obtain a precision regarding the value of these coefficients.
To remedy certain disadvantages of analog processing, it has already been proposed to carry out part of the processing of the chrominance signals in digital form. French patent application No. 2 532 504 filed on Aug. 31, 1983 and entitled "Apparatus for improving the signal to noise ratio of the colour channel in a digital television receiver" describes an apparatus for digitally filtering the two chrominance signals by means of a fine pulse response adaptive filter, whose cutoff frequency is a function of the instantaneous frequency spectrum of each chrominance signal.
This adaptive filtering is more flexible than analog filtering in which the cutoff frequency is fixed. The determination of the cutoff frequency F.sub.c is relatively simple because it is a question of increasing the width of the filter if the energy of the chrominance signal received beyond the cutoff frequency F.sub.c exceeds a certain threshold. However, this filtering criterion is not particularly satisfactory because, in certain cases, it can lead to the filtering of a useful part of the chrominance signal.
Digital filtering of the chrominance signal is also proposed in French patent application No. 2 499 343 entitled "Digital chrominance filter for a television system with digital components and filtering process using said filter". This digital filter is designed for oversampling the chrominance signal, so as to obtain a chrominance signal having a sampling frequency identical to that of the luminance signal. This digital filter is also designed for compensating attenuations introduced by certain elements of the receiver, such as the digital-analog converter, or during the digital coding of the chrominance signal on transmission.